The present invention relates to packaging of electronic circuit devices, such as micro-miniature integrated circuit chips. In particular, the present invention relates to a combined heat-sink and cap assembly for improved dissipation of heat generated by the chips.
Heat dissipation from multi-chip high power modules without adversely affecting the mechanical stability and fatigue life of the solder joints is a problem for the industry.
Current modular designs do not effectively dissipate heat beyond power densities on the order of 25 watts/cm2 without resorting to expensive methods such as use of water or liquid coolant, which minimizes the external thermal resistance and consequently permits a worse internal thermal resistance. The power dissipation limit results from both the internal and external thermal resistance to heat flow.
For air cooled flip chip modules the majority of the thermal resistance is the external resistance. The internal resistance is dominated by the resistance of the chip to cap interface, which is typically filled with a thermal paste. One way to significantly reduce this interface resistance is to replace the paste with solder, but this replaces a compliant interface with a rigid interface, which can cause stresses that adversely affect other parts of the package.
In order to achieve higher power dissipation without compromising fatigue life of solder connections it must be possible to solder the chips to the cap with different gaps between them. It is known that there can be gaps that vary by plus or minus 3 mils between the cap and the chips. The assembly must balance the thermal expansion in the x, y and z directions for the substrate, cap and the chip/solder structure so that the shear and tensile stresses in the solder thermal interface, solder seal, and solder interconnections are kept within acceptable limits.
There are instances of heat sinks directly attached to a single chip and not connected to the substrate because of stresses and fatigue problems, however, this approach is not conducive to, and effective for MCM""s that must be encapsulated to avoid corrosion and excessive fatigue damage of solder interconnections due to high concentrations of oxygen. The best thermal performance today is the dissipation of about 50 watts/cm2, (at the chip level), using air-cooled heat-sinks.
U.S. Pat. No. 4,034,468 4,323,914, 4,607,277, 4,654,966, 4,825,284, 4,920,574, 5,126,919 5,276,586, 5,325,265 and 5,396,403 disclose various types of packages for semi-conductor devices with the provision of heat transfer from the chip to the ambient environment.
A multi-chip heat sink cap assembly is effected using a compliant heat-sink cap structure. The heat-sink cap structure is fabricated so that the heat-sink columns to be arrayed over the chips on the substrate are interconnected via flexible members achieving a unitary structure to overlay the substrate. A portion of the surface of the chip and a portion of the surface of the heat sink column that are to be juxtaposed to one another are coated with a metallized thin film. Solder preforms are placed between the mating metallized surfaces and between the peripheral edge of the heat-sink cap and the substrate. The entire assembly is subject to a solder reflow process to achieve the device of the present invention.
Utilizing a process wherein the assembly is manufactured by manufacturing a compliant heat-sink cap, metallizing the mating surfaces of the chip and the heat sink column and reflowing solder results in the dramatic improvement in the cooling capacity of single and multi-chip modules without impacting the reliability of the chip to substrate interconnects and without the need for an exotic liquid cooling system, such as found in prior art mainframe computers. The apparatus and method according to the present invention overcome the difficulty of reliably attaching a cap/heat-sink to a substrate and chips while insuring that all chip sites are reliably interconnected in a multi-chip module. Thus, heat dissipation by improving the thermal conductive capacity of the modules is accomplished so that higher power chips can be used. Heat dissipation at a much greater rate than is possible with prior art devices can be effected with the method and apparatus of the present invention. This can be on the order of 75 watts/cm2 or greater, (chip power density).
Thus in one aspect the present invention is a method for fabricating a multi-chip module with an integral heat-sink cap combination by, providing a substrate with a plurality of chips fixed to and arrayed in a specific pattern, forming a heat-sink cap comprising a plurality of heat-sink columns, each heat-sink column having a surface adapted to be bonded to an individual chip, the heat-sink column interconnected by flexible members, the heat-sink columns arrayed in an identical pattern to the chips on the substrate, depositing a conductive thin film on at least a portion of the surfaces of the chips and the heat-sink columns that are to be bonded together, assembling the substrate and the heat-sink cap with solder between mating portions of the heat-sink cap and the chips and between the heat-sink cap and the substrate, and reflowing the solder to effect bonding of the heat-sink to the substrate and the chips.
In another aspect the present invention is a multi-chip module with integral heat-sink cap manufactured by providing a substrate with a plurality of chips fixed to and arrayed in a specific pattern, forming a heat-sink cap comprising a plurality of heat-sink columns, each heat-sink column having a surface adapted to be bonded to an individual chip, the heat-sink columns interconnected by flexible members, the heat-sink columns arrayed in an identical pattern to the chips on the substrate, depositing a conductive thin film on at least a portion of the surfaces of the chips and the heat-sink columns that are to be bonded together, assembling the substrate and the heat-sink cap with solder between mating portions of the heat-sink cap and the chips and between the heat-sink cap and the substrate, and reflowing the solder to effect bonding of the heat-sink to the substrate and the chips.